Rasmus played for the Triple-A Gwinnett Braves until he was called up to the Braves on May 18, 2013.[2] He made his major league debut on May 22, 2013, working one and two-thirds innings, and striking out three while giving up home runs to Aaron Hicks and Oswaldo Arcia.[3]

On May 27, 2013, Cory pitched to his older brother Colby Rasmus for the first time in an MLB game. Colby hit a double.[4] This was the first time that one brother pitched to another brother in the MLB since June 13, 2010.[5]

He was optioned back to Gwinnett on May 29 when Jordan Walden was activated from the disabled list.[6]